The present invention relates to sewing apparatus including an arrangement for automatically monitoring the bobbin thread, and also to a bobbin particularly useful in such apparatus.
The known sewing machines are provided with various means for feeding an upper thread to the sewing needle, and other means, including a bobbin, for feeding the lower thread to the sewing needle. Monitoring the condition, particularly breakage, of the upper thread is relatively simple, and several methods are in use today, as described for example in U.S. Pat. No. 3,843,883. However, monitoring the condition of the lower bobbin thread is somewhat more problematical, and although a number of systems have been devised for doing this, the known systems are still not entirely satisfactory. The main reason for this is because of the complexity of the path of the lower thread out of the bobbin, which enables very little room for detection. Various methods for detecting an empty bobbin condition using a light beam are known, for example as described in U.S. Pats. No. 4,237,807 and 4,212,257, and in British Patents No. 1,335,677 and 2,078,798. However, these known techniques are usually of complicated construction and generally do not detect all the conditions of the bobbin thread, including thread exhaustion and thread breakage.
Further, the known sewing machines commonly include various devices for maintaining the thread relatively tight on the bobbin, but the arrangements used are also generally of complicated construction.